High pressure Raman study of intermolecular interactions and Fermi resonance in liquid ethylene carbonate

Signatures of coherent vibrational dynamics in ethylene carbonate

Despite having practical applications in battery technology and serving as a model system for Fermi resonance coupling, ethylene carbonate (EC) receives little direct attention as a vibrational probe in nonlinear vibrational spectroscopy experiments. EC contains a Fermi resonance that is well-characterized in the linear spectrum, and the environmental sensitivity of its Fermi resonance peaks could make it a good molecular probe for two-dimensional infrared spectroscopy (2DIR) experiments. As a model system, we investigate the linear and 2DIR vibrational spectrum of the carbonyl stretching region of ethylene carbonate in tetrahydrofuran. The 2DIR spectrum reveals peak dynamics that evolve coherently. We characterize these dynamics in the context of Redfield theory and find evidence that EC dynamics proceed through coherent pathways, including singular coherence transfer pathways that have not been widely observed in other studies. We find that coherent contributions play a significant role in the observed dynamics of cross-peaks in the 2DIR spectrum, which must be accounted for to extract accurate measurements of early waiting time dynamics.

Aggregation induced spectral splitting and Fermi resonance of Ethylene Carbonate in binary mixture

The vibration band of the ring stretching (ν₁₄), the fundamental ring breathing (ν₁₇) and the Fermi resonance band of carbonyl stretching mixing with the overtone of the ring breathing (ν_{5 +} 2ν₁₇) have been investigated in solid ethylene carbonate (EC) and EC/CH₃CN and EC/CHCl₃ binary mixture. Dimer structure with aggregation-induced spectral splitting model (AIS) was applied to calculate the vibration spectra using the B3LYP-D3/6-311+G (d,p) procedure. The noncoincidence effect (NCE) and concentration induced frequency shifts of the ν₁₄ and ν₅ could be well explained by AIS model based on the dimer structure. Four bands were observed with two in the isotropic and two in the anisotropic Raman spectra and their NCE value decreased with the decrease of EC volume fraction in the binary mixture, and finally disappeared. NCE value and the Fermi resonance constants of EC at different concentrations were calculated from the experimental data.

Temperature-dependent study of Fermi resonance of CH3CN and CH3CN---Li+ complex in CH3CN-LiClO4 mixture by Raman spectroscopy

The Raman spectra of acetonitrile-LiClO₄ mixture solution have been measured in the temperature range 20 to -196 °C at ambient pressure. Detailed Raman spectroscopy analysis revealed that, in acetonitrile-LiClO₄ mixture solution, the liquid CH₃CN transformed into solid phase β at approximately -50 °C, and then into solid phase α at approximately -60 °C. Besides, the Fermi resonance parameters of CH₃CN and CH₃CN---Li⁺ complex at different temperatures were calculated by using the Bertran's equations, respectively. It was found that the Fermi resonance coefficient W of CH₃CN---Li⁺ complex was not sensitive to the variation of temperature from 20 to -45 °C. In the case of CH₃CN, however, the Fermi resonance coefficient W decreased from the temperature of 20 to -196 °C during which a sudden increase was observed at the temperature of -50 °C coinciding with the temperature of phase transition from liquid to solid phase β. Finally, the temperature induced precipitation behavior of LiClO₄ and the structural evolution of CH₃CN on the Fermi resonance have been analyzed.

A small-volume PVTX system for broadband spectroscopic calibration of downhole optical sensors

An instrument is presented that is capable of measuring the optical spectrum (long-wave ultraviolet through short-wave mid-infrared) of fluids under a range of temperature and pressure conditions from ambient pressure up to 138 MPa (20 000 psi) and 422 K (300 °F) using ∼5 ml of fluid. Temperature, pressure, and density are measured in situ in real-time, and composition is varied by adding volatile and nonvolatile components. The stability and accuracy of the conditions are reported for pure ethane, and the effects of temperature and pressure on characteristic regions of the optical spectrum of ethane are illustrated after correction for temperature and pressure effects on the optical cell path length, as well as normalization to the measured density. Molar absorption coefficients and integrated molar absorption coefficients for several vibrational combination bands are presented.

Molecular interactions of organic molecules at the air/water interface investigated by sum frequency generation vibrational spectroscopy

The molecular structure and dynamics of organic molecules at the aqueous interface have attracted a number of investigations owing to their importance and specific nature. However, there are relatively few studies on the direct characterization of the molecular interactions at the air/water interface because they are extremely difficult to measure in experiments. In this study, we use dibutyl ester molecules (R₁CO₂R₂O₂CR₁) as a model of organic molecules, and investigate their molecular structure and interactions using sum frequency generation vibrational spectroscopy. We demonstrate that the molecular interactions can be estimated by measuring the intensity ratio of the symmetric stretching (ν₁) and Fermi resonant bands (2ν₂) of methyl groups. Here, dibutyl ester molecules are widely used as plasticizers in polymers to improve the properties of the plastics and polymers. It is found that the orientation angles of the tailed methyl groups at the air/water interface decrease from 34° to 19° when the chain length of R₂ increases from 0 to 8. The total intermolecular interactions of the dibutyl ester molecules decrease as the chain length of R₂ increases because the van der Waals interactions between the hydrocarbon chains increase, while the hydrogen bond interactions between the carbonyl group and water molecules decrease. Our study demonstrates the stability of ester-based plasticizers in polymers can be well predicted from the intensity ratio of the ν₁ and 2ν₂ bands of methyl group. Such an intensity ratio can be thus used as an effective vibrational optical ruler for characterizing molecular interactions between plasticizers and polymers.

Hydrogen-bond dynamics and Fermi resonance in high-pressure methane filled ice

High-pressure, variable temperature infrared spectroscopy and first-principles calculations on the methane filled ice structure (MH-III) at high pressures are used to investigate the vibrational dynamics related to pressure induced modifications in hydrogen bonding. Infrared spectroscopy of isotopically dilute solutions of H(2)O in D(2)O is employed together with first-principles calculations to characterize proton dynamics with the pressure induced shortening of hydrogen bonds. A Fermi resonance is identified and shown to dominate the infrared spectrum in the pressure region between 10 and 30 GPa. Significant differences in the effects of the Fermi resonance observed between 10 and 300 K arise from the double-well potential energy surface of the hydrogen bond and quantum effects associated with the proton dynamics.

Anisotropy shift and Raman bandwidth studies in carbonyl containing molecule o-chlorobenzaldehyde: role of repulsive forces

The analysis of Raman anisotropy shift as a function of solvent concentration shows the weakening of pair interaction of the molecules due to the influence of solvent-induced perturbations. The present study deals with the effect of dielectric constant of the medium on the non-coincidence effect (anisotropy shift) and the role of van der Waals' volume on the anisotropic Raman bandwidth. The CO stretching vibration of o-chlorobenzaldehyde (OCBD) molecule was studied in various polar and non-polar solvents namely CCl4, CH3CN, C6H5Cl and CH3C6H5. The data on anisotropic bandwidth are interpreted using the van der Waals' volume within the framework of lineshape theory of Bratos and Tarjus, while the Onsager-Fröhlich model on non-coincidence effect has been tested. Our study shows that the repulsive potential of the type e-alphaR is playing an important role in the OCBD-solvent interactions. The vanishing of anisotropy shift (non-coincidence effect) on dilution may be explained on the basis of repulsive forces playing a significant role in solute-solvent interactions.

Vibrational spectra of CCl4: isotopic components and hot bands. Part I

Room temperature (295 degrees K) infrared spectra of CCl4 have been measured covering the 4000-200 cm(-1) region. Bands ascribable to the isotopic components in the first overtone of the asymmetric deformation (2v4) mode and the combination of the symmetric and the asymmetric deformation (v2 + v4) mode regions are observed in agreement with the percentage distribution of various isotopes. From these observations, the first ever estimate of the isotopic band positions in the regions of the v4 and v2 modes at room temperature are obtained. Successful assignment of the isotopic bands in the asymmetric stretching (v3) and combination of the symmetric stretching and asymmetric deformation (v1 + v4) mode regions has been achieved and realistic values of all the v3/(v1 + v4) mode Fermi resonance parameters are obtained. The experimental results suggest that the transitions, where the initial levels are not the ground state, contribute significantly and give rise to a very complicated vibrational spectrum for CCl4 vapour.

The effect of pressure on the bacteriochlorophyll a binding sites of the core antenna complex from Rhodospirillum rubrum

In this paper we examine the effect of pressure on the absorption spectrum and binding site of the core antenna complex from the photosynthetic bacterium Rhodospirillum rubrum. Absorption spectra and Raman spectra in preresonance with the Qy transition of the bacteriochlorophyll a were studied at pressures up to 625 MPa. In agreement with previous work we observe a pressure-induced red shift and broadening of the absorption spectrum. We show that at these pressures the pigments within the protein matrix at room temperature experience little if any distortion, and the hydrogen-bonding network involving the C2 and C9 carbonyl groups of the pigment molecules are undisturbed. Having shown the lack of sensitivity to pressure of the binding site interactions, which are known to modulate the absorption spectrum, we feel that it is relatively safe to attribute the pressure-induced red shift broadly to solvatochromic effects and, in particular, to the modulation of the pigment-pigment interactions by the pressure. This paper represents the first vibrational study of photosynthetic complexes at high pressure and the first application of FT Raman spectroscopy to biological molecules at high pressure.

The effect of high external pressure on DPPC-cholesterol multilamellar vesicles: a pressure-tuning Fourier transform infrared spectroscopy study

We have investigated the effect of incorporation of cholesterol on the barotropic phase behavior of aqueous dispersions of 1,2-dipalmitoylphosphatidylcholine (DPPC) using Fourier-transform infrared spectroscopy (FTIR) in combination with the diamond anvil technique. Infrared spectral parameters, such as the frequencies, intensities, bandshapes and band splittings have been used to detect structural and dynamical changes upon incorporation of cholesterol into the DPPC bilayer. Analysis of these spectral parameters yields information on conformer population, reorientational fluctuations, interchain interaction, hydrogen bonding, interdigitation packing, and phase transformations of the DPPC/cholesterol mixtures. We present FTIR data of aqueous DPPC dispersions at 0, 10, 20, 30, 40, and 50 mol% cholesterol in the pressure range from 0.001 to 20 kbar at two temperatures, 25 degrees C and 55 degrees C. In addition, comprehensive temperature dependent measurements in the range from 20 degrees C to 80 degrees C were performed at ambient pressure. Analysis of the CH2 symmetric and antisymmetric stretching modes yields information of the effect of cholesterol concentration on the phase transition phenomena occurring in the lipid bilayer. Observation of the correlation field splittings of the CH2 bending and rocking modes monitors structural changes and dynamical properties of the lipid mixtures. Cholesterol induces more orientational disorder of the lipid molecules in terms of an increase of the reorientational fluctuations of the molecules and twisting/torsion motions of the acyl chains in the gel phase even at elevated pressures. It therefore appears that one important role of cholesterol is to make the membrane insensitive to changes in external environment, such as high hydrostatic pressure. Increase of pressure leads to a decrease in half width of the C = O band contour of pure DPPC and of DPPC/cholesterol mixtures, especially for cholesterol concentrations equal and higher than 30 mol%, which might be due to a marked increase in free carbonyl groups. At high pressure part of the bound water from the interfacial zone of the membrane is withdrawn. Increase of the cholesterol concentration and increase in pressure have opposite effects on the population of free and hydrated carbonyl ester groups of DPPC in the gel phases.

Pressure-Tuned Fermi Resonance in Ice VII

Fermi resonance was observed between the OH stretch and the overtone of the OH bending modes of HDO molecules contaminated in phase VII of D(2)O ice over the pressure range from 17 to 30 gigapascals. An anharmonic coupling constant, which is related to the potential energy surface on which hydrogen-bonded protons oscillate, was found to range around 50 wave numbers through the resonant pressure range. Its experimentally obtained magnitude and pressure-insensitive behavior will be useful for theoretical studies of the potential energy surface and hence of the nature of hydrogen bonding in ice.